做一些cache的时候,我们不可能把数据库的所有的数据都缓存到内存里面,我们要估计缓存的大小。那么如何查看java对象所占的内存大小呢?本文为大家介绍一下方法,希望可以为您提供帮助!更多内容请关注应届毕业生考试网!
(1)做一些cache的时候,我们不可能把数据库的所有的数据都缓存到内存里面,我们要估计缓存的大小。
(2)内存泄露的时候,我们可以查看某些对象的大小来定位问题,当然还有其他的更有效的方式,比如使用MAT分析dump文件
(3)根据jvm的堆内存设置,我们可以知道最多可以创建多少个对象。
从jdk5开始,提供了Instrumentation API,它有一个叫做getObjectSize()的方法,但是,这个方法存在两个问题:
(1)不可以直接使用。必须要实现一个Instrumentation Agent,还得放到jar包里面。
(2)它只能返回单个对象的大小,不能返回内部包含的.子对象的大小。
关于第一个问题,很好解决,在任何一个类里面声明一个"premain"方法,就可以把这个类做成是一个agent:
public class SizeOfAgent {
static Instrumentation inst;
/** initiALizes agent */
public static void premain(String agentArgs, Instrumentation instP) {
inst = instP;
}
}
jvm在启动的时候会调用premain()方法,同时会传递Instrumentation这个对象实例,要告诉jvm Instrumentation agent所在的类,需要把这个类打到jar包里面,
然后在这个文件设置一些属性:
Premain-Class: OfAgent
Boot-Class-Path:
Can-Redefine-Classes: false
java应用在启动的时候,指定-javaagent参数:
java -javaagent: <Your main class>
拿到Instrumentation这个实例以后,就可以调用sizeOf()方法了:
public class SizeOfAgent {
static Instrumentation inst;
// ...
public static long sizeOf(Object o) {
return bjectSize(o);
}
}
然后可以使用反射来获取子对象的大小。
完整的代码如下:
package ;
import rumentation;
import y;
import d;
import fier;
import tityHashMap;
import ;
import k;
public class SizeOfAgent {
static Instrumentation inst;
/** initializes agent */
public static void premain(String agentArgs, Instrumentation instP) {
inst = instP;
}
/**
* Returns object size without member sub-objects.
*
* @param o
* object to get size of
* @return object size
*/
public static long sizeOf(Object o) {
if (inst == null) {
throw new IllegalStateException(
"Can not access instrumentation environment.n"
+ "Please check if jar file containing SizeOfAgent class is n"
+ "specified in the java's "-javaagent" command line argument.");
}
return bjectSize(o);
}
/**
* Calculates full size of object iterating over its hierarchy graph.
*
* @param obj
* object to calculate size of
* @return object size
*/
public static long fullSizeOf(Object obj) {
Map<Object, Object> visited = new IdentityHashMap<Object, Object>();
Stack<Object> stack = new Stack<Object>();
long result = internalSizeOf(obj, stack, visited);
while (!pty()) {
result += internalSizeOf((), stack, visited);
}
r();
return result;
}
private static boolean skipObject(Object obj, Map<Object, Object> visited) {
if (obj instanceof String) {
// skip interned string
if (obj == ((String) obj)rn()) {
return true;
}
}
return (obj == null) // skip visited object
|| ainsKey(obj);
}
private static long internalSizeOf(Object obj, Stack<Object> stack,
Map<Object, Object> visited) {
if (skipObject(obj, visited)) {
return 0;
}
(obj, null);
long result = 0;
// get size of object + primitive variables + member pointers
result += Of(obj);
// process all array elements
Class clazz = lass();
if (ray()) {
if (ame()th() != 2) {// skip primitive type array
int length = ength(obj);
for (int i = 0; i < length; i++) {
((obj, i));
}
}
return result;
}
// process all fields of the object
while (clazz != null) {
Field[] fields = eclaredFields();
for (int i = 0; i < th; i++) {
if (!atic(fields[i]odifiers())) {
if (fields[i]ype()imitive()) {
continue; // skip primitive fields
} else {
fields[i]ccessible(true);
try {
// objects to be estimated are put to stack
Object objectToAdd = fields[i](obj);
if (objectToAdd != null) {
(objectToAdd);
}
} catch (IllegalAccessException ex) {
assert false;
}
}
}
}
clazz = uperclass();
}
return result;
}
}
然后我们可以做一个测试:
public class Test {
static class Person{
private int id;
private String name;
private String address;
public Person(int id, String name, String address) {
= id;
= name;
ess = address;
}
}
public static void main(String[] args) throws Exception {
Person p = new Person(12, "xujsh","bj");
long size = SizeOf(p);
tln(size);
}
}
切换到命令行:
D:workspaceobjsizesrc>java -version
java version "1.6.0_22"
Java(TM) SE Runtime Environment (build 1.6.0_22-b04)
Java HotSpot(TM) Client VM (build 17.1-b03, mixed mode, sharing)
D:workspaceobjsizesrc>javac com/bj58/test/*
D:workspaceobjsizesrc>jar -cvfm com/bj58/test/*
标明清单(manifest)
增加:com/bj58/test/s(读入= 3119) (写出= 1698)(压缩了 45%)
增加:com/bj58/test/(读入= 3147) (写出= 1204)(压缩了 61%)
增加:com/bj58/test/Test$s(读入= 442) (写出= 305)(压缩了 30%)
增加:com/bj58/test/s(读入= 692) (写出= 441)(压缩了 36%)
增加:com/bj58/test/(读入= 509) (写出= 290)(压缩了 43%)
D:workspaceobjsizesrc>java -javaagent:
24
:
Manifest-Version: 1.0
Main-Class:
Premain-Class: OfAgent
Boot-Class-Path:
Can-Redefine-Classes: false
【注意】文件的格式要求比较严格,每一行要满足:key:空格value回车
如何在web应用程序里面使用呢?
以我的tomcat为例,
(1)把上传tomcat的lib目录下面
(2)修改:
添加一行:
JAVA_OPTS="$JAVA_OPTS -javaagent:$CATALINA_HOME/lib/" //这一行是新添加的
if [ -z "$LOGGING_MANAGER" ]; then
JAVA_OPTS="$JAVA_OPTS ger=sLoaderLogManager"
else
JAVA_OPTS="$JAVA_OPTS $LOGGING_MANAGER"
fi
(3)在应用里面添加一个controler:
@Path(value = "/api/size")
@GET
public ActionResult size() {
Map<Long, List<Long>> map = otindexBaidu();
long size = SizeOf(map);
return new ApiActionResult("size:"+size);
}